Which product and component would you like to have described?
or choose from the following selections
Shared components used by Firefox and other Mozilla software, including handling of Web content; Gecko, HTML, CSS, layout, DOM, scripts, images, networking, etc. Issues with web page layout probably go here, while Firefox user interface issues belong in the Firefox product. (More info)
For bugs in Firefox Desktop, the Mozilla Foundation's web browser. For Firefox user interface issues in menus, bookmarks, location bar, and preferences. Many Firefox bugs will either be filed here or in the Core product. Bugs for developer tools (F12) should be filed in the DevTools product. (more info)
Fenix (Firefox for Android) is a new Android browser built using Android Components that re-imagines how a mobile browser would look if it was built from scratch to demonstrate better design for typical mobile use cases (more info).
Firefox for iOS is the Firefox mobile experience developed for the iOS platform.
For bugs in Firefox Focus for Android. Bugs in Firefox Focus for iOS should be filed on GitHub
For bugs in Firefox DevTools, the developer tools within the Firefox web browser. This includes issues about the user interface of the toolbox, special pages such as about:debugging and about:devtools, and developer-related APIs.
File feature requests and bugs related to the WebExtension add-on API.
The Mozilla Toolkit is a set of APIs, built on top of Gecko, which provide advanced services to XUL applications. These services include Profile Management, Chrome Registration, Browsing History, Extension and Theme Management, Application Update Service, and Safe Mode. (More info)
The GeckoView components for Android
Translation, spelling and other errors in language packs and localized builds (more info)
Requests for the Firefox User Research Team.
For bugs in the Thunderbird mail client front-end. Bugs in the back-end (e.g. protocols) should be filed under MailNews Core. Bugs in Thunderbird's built in calendar and chat features should be filed in the Calendar or Chat components respectively.
An all-in-one internet application suite, including web browser, e-mail and newsgroup client, and HTML composer. (more info)
Other Mozilla products which aren't listed here